As household and other wastes gradually decompose in commercial landfills, significant quantities of gases are generated as decomposition products. These include hydrocarbons, particularly methane, as well as carbon dioxide, nitrogen, water vapor, and other gases and hydrocarbons. If captured and recovered from the landfill site, the methane generated during landfill decomposition represents a potentially valuable energy or feedstock source, particularly if the methane is refined or purified in such a manner as to meet the quality requirements to be accepted for delivery in natural gas pipelines to customers as a substitute for natural gas obtained from drilled natural gas wells.
To be combusted and used as an energy or feedstock source, however, the methane must first be separated from the other landfill gases and substantially purified. Therefore, there is a need for improved methods and systems to separate landfill gases, particularly methane, from other components of landfill gas streams so as to provide a substantially purified methane stream which may be either combusted and used as an energy source or used as a feedstock for the manufacture of other chemicals or substances. SUMMARY
In a first aspect, the present disclosure provides a method for separating and purifying a landfill gas stream. In one embodiment, the method includes a step of collecting a crude landfill gas stream which includes at least methane, carbon dioxide, oxygen, and nitrogen gases. The method also includes a step of separating the crude landfill gas stream into at least an intermediate landfill gas stream and a first waste gas stream using a membrane separation unit, wherein the intermediate landfill gas stream is enriched in methane and depleted in carbon dioxide relative to the crude landfill gas stream. The method further includes a step of separating the intermediate landfill gas stream into at least a final landfill gas stream and a second waste gas stream using a pressure swing adsorption separation unit, wherein the final landfill gas stream is enriched in methane and depleted in carbon dioxide, oxygen, and nitrogen relative to the intermediate landfill gas stream.
In certain embodiments according to the present disclosure, the crude landfill gas stream preferably includes from about 45 to about 55 mole percent methane, from about 20 to about 40 mole percent carbon dioxide, and from about 5 to about 20 mole percent nitrogen. The final landfill gas stream preferably includes at least about 96 mole percent methane, no more than about 1 mole percent carbon dioxide, and no more than about 4 mole percent nitrogen.
In certain embodiments according to the present disclosure, the first waste gas stream includes volatile organic compounds, and the method preferably also includes a step of treating the first waste gas stream in a thermal oxidizer unit to substantially destroy the volatile organic compounds therein so that the first waste gas stream may be released into the atmosphere The second waste gas stream may also include volatile organic compounds, in other embodiments according to the present disclosure, and the method preferably also includes a step of treating the second waste gas stream in a thermal oxidizer unit to substantially destroy the volatile organic compounds therein so that the second waste gas stream may be released into the atmosphere.
In some embodiments according to the present disclosure, at least a portion of the crude landfill gas stream is separated into a first recycle gas stream using the membrane separation unit, and this first recycle gas stream is recombined with the crude landfill gas stream. In certain embodiments, this first recycle gas stream is preferably from about 30 to about 40 mole percent of the crude landfill gas stream.
In certain other embodiments according to the present disclosure, at least a portion of the intermediate landfill gas stream is separated into a second recycle gas stream using the pressure swing adsorption separation unit, and this second recycle gas stream is recombined with the crude landfill gas stream. In certain embodiments, the second recycle gas stream is preferably from about 17 to about 22 mole percent of the intermediate landfill gas stream.
In still other embodiments according to the present disclosure, the crude landfill gas stream may also include hydrogen sulfide gas or other compounds of sulfur. In these embodiments, the method preferably also includes a step of removing at least a portion of the hydrogen sulfide gas or other compounds of sulfur from the crude landfill gas stream before the crude landfill gas stream is separated using the membrane separation unit.
In certain embodiments according to the present disclosure, the crude landfill gas stream further includes oxygen, water vapor, and volatile organic compounds and wherein the final landfill gas stream is depleted in oxygen, water vapor, and volatile organic compounds relative to the crude landfill gas stream.
In another aspect, the present disclosure provides a system for separating and purifying a landfill gas stream. In one embodiment the system includes a feed gas conduit in flow communication with a crude landfill gas stream which includes at least methane, carbon dioxide, oxygen, and nitrogen gases. The system also includes a membrane separation unit in flow communication with the feed gas conduit for separating the crude landfill gas stream into at least an intermediate landfill gas stream and a first waste gas stream, wherein the intermediate landfill gas stream is enriched in methane and depleted in carbon dioxide relative to the crude landfill gas stream. The system further includes a pressure swing adsorption separation unit in flow communication with the membrane separation unit for receiving the intermediate landfill gas stream therefrom and separating the intermediate landfill gas stream into at least a final landfill gas stream and a second waste gas stream, wherein the final landfill gas stream is enriched in methane and depleted in carbon dioxide, oxygen, and nitrogen relative to the intermediate landfill gas stream.
The system may also include a thermal oxidizer unit. In certain embodiments according to the present disclosure, the thermal oxidizer unit is preferably in flow communication with the membrane separation unit for receiving the first waste gas stream, which may include volatile organic compounds. The thermal oxidizer unit substantially destroys the volatile organic compounds in the first waste gas stream so that the first waste gas stream may be released into the atmosphere. In certain other embodiments according to the present disclosure, the thermal oxidizer unit is preferably in flow communication with the pressure swing adsorption unit for receiving the second waste gas stream, which may also include volatile organic compounds. The thermal oxidizer unit substantially destroys the volatile organic compounds in the second waste gas stream so that the second waste gas stream may be released into the atmosphere.
In certain embodiments according to the present disclosure, the system also includes a recycle conduit in flow communication with at least the membrane separation unit. At least a portion of the crude landfill gas stream is preferably separated into a first recycle gas stream using the membrane separation unit and this first recycle gas stream is recombined with the crude landfill gas stream via the recycle conduit. In certain embodiments, this first recycle gas stream is preferably from about 30 to about 40 mole percent of the crude landfill gas stream.
In certain embodiments according to the present disclosure, the system also includes a recycle conduit in flow communication with at least the pressure swing adsorption unit. At least a portion of the intermediate landfill gas stream is preferably separated into a second recycle gas stream using the pressure swing adsorption separation unit and this second recycle gas stream is recombined with the crude landfill gas stream via the recycle conduit. In certain embodiments, this second recycle gas stream is preferably from about 17 to about 22 mole percent of the intermediate landfill gas stream.
In certain embodiments according to the present disclosure, the crude landfill gas stream may also include hydrogen sulfide gas. In these embodiments, the system may also include a hydrogen sulfide separation unit in flow communication with the crude landfill gas stream for removing at least a portion of the hydrogen sulfide gas from the crude landfill gas stream before the crude landfill gas stream is separated using the membrane separation unit.